Network node, method, and mobile terminal for providing voice calls to a mobile terminal in a packet-switched-only network

ABSTRACT

A network node, method, and mobile terminal for providing circuit-switched (CS) voice calls to a mobile terminal operating in a packet-switched (PS)-only network. A control node such as an EPC node in an SAE/LTE PS-only network is modified to mimic the messaging functionality of a Serving GPRS Service Node (SGSN). The EPC node is connected to an MSC/VLR in a CS network through a Gs interface. The EPC node registers the terminal with the MSC/VLR and provides a VLR TMSI to the terminal. When the MSC/VLR receives an incoming CS voice call for the terminal, the MSC/VLR notifies the EPC node, which sends a paging message to the terminal. To set up the CS voice call, the terminal sends a paging response directly to the MSC/VLR.

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BACKGROUND OF THE INVENTION

This invention relates to communication systems. More particularly, andnot by way of limitation, the invention is directed to a network node,method, and mobile terminal for providing voice calls to a mobileterminal operating in a packet-switched-only network.

Wireless networks today, such as second-generation (2G) GSM networks,third-generation (3G) networks, and Wireless Local Area Networks (WLANs)support Packet-Switched (PS) services and/or Circuit-Switched (CS)services. A user in a 2G or 3G network may use either CS services suchas speech, or PS services such as Internet Access. The availability ofCS and PS services also depends on the terminal. Some devices such asLaptop PC-cards are PS-only devices, while other devices such as olderGSM phones without GPRS support are CS-only devices. A user in a PS-onlynetwork may use only PS services, although Voice-over-IP (VoIP) isemerging. New wireless networks such as Wimax support only PS and notCS. New standards for PS-only networks are also being investigated bythe Third Generation Partnership Project (3GPP) in a feasibility studyfor System Architecture Evolution/Long Term Evolution (SAE/LTE). The SAEportion of this study is focused on a new core network, while the LTEportion of the study is focused on a new radio network.

General information regarding 2G and 3G networks may be found in thetechnical specification 3GPP TS 23.060, v.6.13.0. Further informationabout the Gs interface may be found in the technical specification 3GPPTS 29.018, v. 6.4.0. Further information about SAE/LTE networks may befound in the technical specification 3GPP TS 23.882, v. 1.2.3. Each ofthese technical specifications is hereby incorporated herein byreference.

Many users, however, prefer CS voice service because the quality of CSvoice service is in many cases superior to PS voice quality. Operatorsmay also prefer CS voice services because operators must heavily investto provide VoIP services.

Some existing networks contain both a CS part and a PS part of thenetwork. An interface known as the Gs interface provides co-ordinationbetween a Mobile Switching Center/Visitor Location Register (MSC/VLR),which handles CS traffic in the CS part of the network, and a ServingGPRS Service Node (SGSN), which handles PS traffic in the PS part of thenetwork. The Gs interface is used by the MSC/VLR for paging the mobileterminal for CS services, for exchanging location information betweenthe CS part and the PS part, and for conveying some CS-relatedprocedures through the SGSN. The Gs interface makes it possible for amobile terminal supporting both CS and PS services to connect to bothservices via the PS domain. The Gs interface enables a mobile terminalusing a combined CS and PS network to send some messages only to the PSpart to save radio transmissions. The PS part then informs the CS partof needed information through signaling to ensure that both the CS andPS parts of the network have the same information about the mobileterminal.

When a mobile terminal is operating in a PS-only network, there areseveral reasons why the mobile terminal cannot receive CS voice calls.First, there is no mechanism for registering the mobile terminal with anMSC. Second, even if the mobile terminal was registered with an MSC/VLR,there is no signaling mechanism for forwarding paging messages from theMSC/VLR to the mobile terminal. Thus, the mobile terminal cannot bealerted when a CS voice call is incoming. Additionally, the mobileterminal itself lacks the functionality for using more than one radiotechnology at the same time. Although some high-end mobile terminals arecapable of supporting more than one network technology, they cannot usemore than one radio technology at the same time because batteryconsumption or interference issues make it technically impossible.Low-end terminals, where cost is an important issue, are often notdesigned with this capability due to cost constraints. Thus, theterminal is restricted to using either PS-only data services or CS-onlyvoice services, but not both at the same time. Therefore, if the mobileuser is using a PS-only data service, the user cannot receive any CSvoice calls.

It should also be noted that networks providing both CS and PS services(such as GSM or WCDMA), generally have better coverage than PS-onlynetworks (such as WLAN). However, PS-only networks generally offerhigher bandwidths, perhaps even 100 times higher, than networksproviding both CS and PS services. This higher bandwidth can be veryuseful for applications such as IPTV, video, downloading from theInternet, and the like. When a mobile terminal is using a PS service ina PS-only network, it is technically feasible to maintain the PS servicewhen moving to a combined CS/PS network, if the two networks areoperated by the same operator, and if the operator has chosen toimplement the network in this way. In the combined CS/PS network, thesame radio access technology is utilized for both CS and PS services,and the mobile terminal can receive CS voice calls while running the PSservice. However, in most cases, some PS bandwidth capability will belost. From the bandwidth perspective, it would be beneficial for theterminal to continue to use the PS-only network, but the drawback isthat any incoming CS calls to the mobile terminal are lost.

What is needed in the art is a network node, method, and mobile terminalfor providing voice calls to a mobile terminal that overcomes thedisadvantages of the prior art. The present invention provides such anetwork node, method, and mobile terminal.

BRIEF SUMMARY OF THE INVENTION

The present invention is a network node, method, and mobile terminal forproviding voice calls to a mobile terminal operating in apacket-switched-only network. From the user's perspective, the inventionprovides the advantage that it enables a mobile terminal to receive CSvoice calls when using a PS-only network. From the network operator'sperspective, the invention enables the network operator to provide CSvoice calls when the mobile users are using PS-only networks. Theoperator can therefore provide PS-only network services without havingto fully implement VoIP services.

In one aspect, the present invention is directed to a method ofproviding a circuit-switched voice call to a mobile terminal operatingin a packet-switched-only network. The method includes mimicking, in acontrol node in the packet-switched-only network, the messagingfunctionality of a Serving GPRS Service Node (SGSN); connecting thecontrol node to a mobile switching center/visitor location register(MSC/VLR) in a circuit-switched network through a Gs interface; andforwarding by the control node, a paging message received from theMSC/VLR. The paging message is forwarded to the mobile terminal fordirect response to the MSC/VLR.

In another aspect, the present invention is directed to a control nodein a packet-switched-only network. The control node includes apacket-switched-only communication unit for wirelessly communicatingwith a mobile terminal; a SGSN mimic unit for mimicking the messagingfunctionality of an SGSN; and a Gs interface connection from the SGSNmimic unit to an MSC/VLR in a circuit-switched network.

In another aspect, the present invention is directed to a mobileterminal for receiving a CS voice call while operating in a PS-onlynetwork. The mobile terminal includes a packet-switched communicationprotocol stack for sending and receiving messages with a control node inthe PS-only network; a circuit-switched communication protocol stack forsending and receiving messages with an MSC/VLR in a CS network; and acontroller for determining when to use the packet-switched communicationprotocol stack and when to use the circuit-switched communicationprotocol stack.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING

In the following, the essential features of the invention will bedescribed in detail by showing preferred embodiments, with reference tothe attached figures in which:

FIG. 1 is a signaling diagram illustrating the flow of messages betweenvarious network entities when a mobile terminal registers with a PS-onlynetwork in an exemplary embodiment of the system and method of thepresent invention;

FIG. 2 is a signaling diagram illustrating the flow of messages betweenvarious network entities when a mobile terminal performs a Tracking AreaUpdate with a PS-only network in an exemplary embodiment of the systemand method of the present invention;

FIG. 3 is a signaling diagram illustrating the flow of messages betweenvarious network entities when a mobile terminal receives a CS voice callwhile operating in a PS-only network in an exemplary embodiment of thesystem and method of the present invention;

FIG. 4 is a signaling diagram illustrating the flow of messages betweenvarious network entities when a mobile terminal receives a CS voice callwhile actively involved in data transmission in a PS-only network in anexemplary embodiment of the system and method of the present invention;

FIG. 5 is a simplified block diagram of an exemplary embodiment of a PSnetwork node as taught by the present invention; and

FIG. 6 is a simplified block diagram of an exemplary embodiment of amobile terminal as taught by the present invention.

DETAILED DESCRIPTION OF THE INVENTION

The present invention enables a mobile terminal to receive CS voicecalls when operating in a PS-only network. This feature is enabled byadding to one or several nodes in the PS-only network, the functionalityto act as an SGSN towards a combined CS/PS network with both CS and PSfunctionality, such as 2G or 3G networks. One or several nodes in thePS-only network then function as an SGSN proxy towards the combinedCS/PS network, while requiring minimal or no changes in the combinedCS/PS network.

FIG. 1 is a signaling diagram illustrating the flow of messages betweenvarious network entities when a mobile terminal (MT) 11 registers with aPS-only network in an exemplary embodiment of the system and method ofthe present invention. It is assumed that the MT is capable of utilizinga combined PS/CS network (such as a 3G network), and is also capable ofutilizing a different PS-only network (such as, for example, an SAE/LTEnetwork). However, the MT is not able to use both networks at the sametime. The MT communicates with an SAE/LTE Evolved Packet Core (EPC) node12 over a PS-only Evolved Radio Access Network (RAN) 13. The term EPCdenotes the Core Network and the included nodes needed for SAE. The EPCnode, in turn, communicates with a Mobile Switching Center/VisitorLocation Center (MSC/VLR) 14 in the combined PS/CS network over the GsInterface 15. To communicate directly with the MSC/VLR, the MT utilizesa combined (2G or 3G) RAN 16.

At step 17, the MT 11 sends a registration message includingauthentication information to the EPC node 12. The MT may also includecertain parameters, which enable the EPC node to mimic an SGSN. In atypical 2G/3G network registration, the MT would register with an SGSNby sending a combined GPRS and IMSI Attach message to the SGSN.Therefore, to enable the EPC node to mimic or emulate an SGSN, the MTmay send the same parameters normally included in the GPRS and IMSIAttach message.

At step 18, the EPC node 12 utilizes the Gs Interface 15 to send aLocation Update Request message to the MSC/VLR 14. Included in themessage are parameters enabling the EPC node to mimic or emulate anSGSN. At step 19, the MSC/VLR sends a Location Update Accept message tothe EPC node. This message acknowledges the Location Update Request andincludes a temporary identifier (e.g., VLR TMSI) for the MT. At step 20,the EPC node sends a Registration Confirm message to the MT. Thismessage acknowledges the Registration message 17 and includes severalparameters including the VLR TMSI received from the MSC/VLR. The MTstores the VLR TMSI, which is needed for communications with theMSC/VLR.

FIG. 2 is a signaling diagram illustrating the flow of messages betweenvarious network entities when a mobile terminal performs a Tracking AreaUpdate with a PS-only network in an exemplary embodiment of the systemand method of the present invention. If the MT 11 changes Tracking Areasin the PS-only network, the MT reports this fact in a Tracking AreaUpdate (TAU) message 22, which is sent to the EPC node 12. The change ofTracking Areas may or may not equate to a change of Location Areas inthe 2G or 3G network. In 2G networks such as GSM, MTs are tracked inLocation Areas (LAs) and Routing Areas (RAs). In UMTS (3G) networks, MTsmay be tracked in UTRAN Registration Areas (URAs) as well as LAs andRAs. The term “Tracking Area” is used as a generic name for URAs, LAs,and RAs. The EPC node is programmed with information for translatingTracking Areas into the equivalent Location Areas (for example, a TA/LAlookup table), and at step 23, the EPC node determines whether theLocation Area has also changed. If the Location Area has changed, theMSC/VLR 14 must be informed. Therefore, at step 24, the EPC nodeutilizes the Gs Interface 15 to send a Location Update Request messageto the MSC/VLR. Included in this message are the parameters necessary tomimic an SGSN. At step 25, the MSC/VLR sends a Location Update Acceptmessage back to the EPC node. At step 26, the EPC node sends a TAUConfirm message back to the MT. Note that this message is sent toacknowledge the TAU message 22 regardless of whether the Location Areawas changed.

FIG. 3 is a signaling diagram illustrating the flow of messages betweenvarious network entities when a mobile terminal receives a CS voice callwhile operating in a PS-only network in an exemplary embodiment of thesystem and method of the present invention. Steps 17 through 20illustrate the registration process as described above in connectionwith FIG. 1. At step 30, the MSC/VLR 14 receives an incoming CS call forthe MT 11. At step 31, the MSC/VLR utilizes the Gs interface 15 to senda Page message to the EPC node 12 identifying the MT. At step 32, theEPC node sends the equivalent of a Paging Request message to the MT.Alternatively, this message may be an end-user message notifying the enduser to manually initiate the change of radio network usage from thePS-only SAE/LTE network to the 2G/3G network. At step 33, the MTperforms the procedures to switch from the PS-only SAE/LTE network tothe 2G/3G network. At step 34, the MT sends a Paging Response messagedirectly to the MSC/VLR. Thereafter, at step 35, normal CS call setupprocedures are performed between the MT and the MSC/VLR to accept andreceive the incoming CS call.

It should be understood that the Paging Request message, which the MTrecognizes as a CS-related message, is different from the PS-relatedmessage that the EPC node uses to invite the MT to participate in a PSdata session. The MT recognizes the difference and sends the response toeither the MS/VLR or the EPC node accordingly.

FIG. 4 is a signaling diagram illustrating the flow of messages betweenvarious network entities when a mobile terminal receives a CS voice callwhile actively involved in data transmission in a PS-only network in anexemplary embodiment of the system and method of the present invention.Steps 17 through 20 illustrate the registration process as describedabove in connection with FIG. 1. At step 29, a PS session is establishedbetween the MT 11 and the EPC node 12. If the MT 11 is actively involvedin data transmission in a packet session in the PS-only network at thetime the notification of an incoming CS voice call is received at step30, procedures may be utilized to also move the packet session from thePS-only network to the combined PS/CS network. As a result, the mobileend user is able to continue the packet session in the combined PS/CSnetwork (although with less bandwidth), while also being able to receivethe CS call. In one embodiment, when the MT receives the Paging Requestmessage 32, the MT answers by sending both a Page Response message 34 tothe MSC/VLR 14, and a Routing Area Update message 36 to its SGSN 37 toannounce its presence in the 2G/3G PS network. Thereafter, at step 35,normal CS call setup procedures are performed between the MT and theMSC/VLR to accept and receive the incoming CS call. Likewise, normal PSsession establishment procedures 38 are performed between the MT and theSGSN to reestablish the data session in the 2G/3G PS network.

FIG. 5 is a simplified block diagram of an exemplary embodiment of a PSnetwork node as taught by the present invention. In the illustratedembodiment, the network node is an EPC node 12 in an SAE/LTE network.Network nodes in other types of PS-only networks may also be utilized.The EPC node includes a PS-only communication unit 41, whichcommunicates with the MT 11 operating in the PS-only network. ThePS-only communication unit receives and sends the various messages 17,20, 22, and 26 shown and described in FIGS. 1-4. When the PS onlycommunication unit receives the Registration message 17, itauthenticates the MT with an Authentication Unit 40. The PS-onlycommunication unit then forwards message parameters to an SGSN mimicunit 42 enabling the SGSN mimic unit to mimic SGSN signaling over the Gsinterface 15 to the MSC/VLR 14. In the illustrated example, theforwarded parameters are the parameters from a GPRS/IMSI Attach message.The SGSN mimic unit then sends the Location Update Request message 18 tothe MSC/VLR 14. The MSC/VLR returns the Location Update Accept message19, which as noted earlier, includes the VLR TMSI. The SGSN mimic unitforwards the VLR TMSI to the PS-only communication unit, which sends theVLR TMSI to the MT 11 in the Registration Confirm message 20.

When the EPC node 12 receives a TAU message 22 from the MT 11, thePS-only communication unit 41 forwards the TAU to a TA/LA lookup table43. The TA/LA lookup table determines whether the change of TrackingArea in the TAU message also resulted in a change of Location Area. Ifthe Location Area did not change, the TA/LA lookup table reports thisfact at 44 to the PS-only communication unit 41, which sends the TAUconfirm message 26 to the MT. If the change of Tracking Area alsoresulted in a change of Location Area, the TA/LA lookup table reportsthis fact at 45 to the SGSN mimic unit 42. The SGSN mimic unit uses theGs interface 15 to send the Location Update Request message 18 to theMSC/VLR 14. The MSC/VLR returns the Location Update Accept message 19 tothe EPC node, and any new VLR TMSI information is reported to the MT inthe TAU Confirm message 26.

When the MSC/VLR 14 receives a CS voice call for the MT 11, the MSC/VLRsends the Page message 31 over the Gs interface 15 to the EPC node 12.The Page message includes an identifier for the MT such as the MT'sIMSI. The SGSN mimic unit 41 passes the Page to the PS-onlycommunication unit 41, which sends the Paging Request 32 to the MT. TheMT then sends the Paging Response directly to the MSC/VLR 14.

FIG. 6 is a simplified block diagram of an exemplary embodiment of amobile terminal (MT) 11 as taught by the present invention. When the MTis operating in a PS-only network, a controller 51 controls a PScommunication protocol stack 52, which sends and receives messages 17,20, 22, and 26 through a transceiver (TX/RX) 53 and an antenna 54. Whenthere is an incoming CS voice call for the MT, the EPC node 12 may sendan end-user message to the MT notifying the end user to manuallyinitiate a change of radio network usage from the PS-only network to a2G/3G network supporting CS calls. The controller may notify the userthrough a speaker 55 or a visual display 56. The user may indicate thedesire to accept the CS voice call through a keypad 57. The controllerthen retrieves the VLR TMSI from memory 58 and causes a CS communicationprotocol stack 59 to construct and send the Paging Response 34 utilizingthe transceiver 53 and the antenna 54.

In an alternative embodiment, the MT is capable of automaticallyswitching to the 2G/3G network. When the MT receives the Paging Request32 indicating that there is an incoming CS voice call for the MT, thecontroller automatically switches from the PS communication protocolstack 52 to the CS communication protocol stack 58 to construct and sendthe Paging Response 34 utilizing the transceiver 53 and the antenna 54.

The controller 51 recognizes the Paging Request message as being aCS-related message, and thus uses the CS communication protocol stack 58to send the Paging Response to the MSC/VLR 14. The controller alsorecognizes when the EPC node 12 sends a PS-related invitation toparticipate in a PS data session. In this case, the controller uses thePS communication protocol stack 52 to send a response to the EPC node.

If the MT 11 is actively involved in data transmission in a packetsession in the PS-only network at the time the notification of anincoming CS voice call is received, the procedures discussed above inconnection with FIG. 4 may be utilized to also move the packet sessionfrom the PS-only network to the 2G/3G network. As a result, the mobileend user is able to continue the packet session in the combined PS/CSnetwork (although with less bandwidth), while also being able to receivethe CS call. When the MT receives the Paging Request message 32, thecontroller retrieves the VLR TMSI from memory 58 and causes the CScommunication protocol stack 59 to send a Page Response message 34 tothe MSC/VLR 14. The controller also sends a Routing Area Update message36 to the MT's SGSN 37 to announce its presence in the 2G/3G PS network.Thereafter, normal CS call setup procedures are performed between the MTand the MSC/VLR to accept and receive the incoming CS call. Likewise,normal PS session establishment procedures are performed between the MTand the SGSN to reestablish the data session in the 2G/3G PS network.

Although preferred embodiments of the present invention have beenillustrated in the accompanying drawings and described in the foregoingDetailed Description, it is understood that the invention is not limitedto the embodiments disclosed, but is capable of numerous rearrangements,modifications, and substitutions without departing from the scope of theinvention. The specification contemplates any all modifications thatfall within the scope of the invention defined by the following claims.

What is claimed is:
 1. A method of providing a circuit-switched (CS)voice call to a mobile terminal operating in a packet-switched-onlySystem Architecture Evolution/Long Term Evolution (SAE/LTE) network,said method comprising: in a control node in the SAE/LTE network,mimicking the messaging functionality of a Serving GPRS Service Node(SGSN); connecting the control node in the SAE/LTE network to a mobileswitching center/visitor location register (MSC/VLR) in a CS networkthrough a Gs interface; receiving by the control node, a paging messagefrom the MSC/VLR utilizing the Gs interface; and sending an equivalentof a CS paging request message from the control node to the mobileterminal in the SAE/LTE network, the equivalent of the CS paging requestmessage indicating the MSC/VLR has received an incoming CS voice callfor the mobile terminal, wherein the equivalent of the CS paging requestmessage causes the mobile terminal to send a CS paging response directlyto the MSC/VLR.
 2. The method according to claim 1, further comprising,after the connecting step, the steps of: registering the mobile terminalwith the MSC/VLR by sending a first message from the control node in theSAE/LTE network to the MSC/VLR, said first message including parametersthat mimic parameters sent by a Serving GPRS Service Node (SGSN);receiving by the control node via the Gs interface, an identifier forthe MSC/VLR; and forwarding the identifier for the MSC/VLR to the mobileterminal.
 3. The method according to claim 2, further comprising:receiving by the control node in the SAE/LTE network, a request from theSAE/LTE network to establish a data session with the mobile terminal;and forwarding the request to the mobile terminal for direct response tothe control node.
 4. The method according to claim 1, furthercomprising: performing CS call setup procedures in the CS network to setup the CS voice call with the mobile terminal.
 5. The method accordingto claim 1, wherein the MSC/VLR is in a combined CS/PS network that alsoincludes an SGSN, and the mobile terminal is actively involved in a PSdata session in the SAE/LTE network when the paging message is received,said method further comprising: sending a paging response message fromthe mobile terminal to the MSC/VLR; performing CS call setup proceduresin the combined CS/PS network to set up the CS voice call with themobile terminal; sending a routing area update message from the mobileterminal to the SGSN, said routing area update message announcing themobile terminal's presence in the combined CS/PS network; and performingPS session establishment procedures in the combined CS/PS network toreestablish the PS data session with the mobile terminal.
 6. The methodaccording to claim 2, further comprising: receiving by the control nodein the SAE/LTE network, a tracking area update message from the mobileterminal, the tracking area update message indicating that the mobileterminal has changed tracking areas in the SAE/LTE network; determiningby the control node in the SAE/LTE network, whether the mobile terminalhas also changed location areas associated with the CS network; and ifthe mobile terminal has also changed location areas: sending a locationupdate message from the control node in the SAE/LTE network to theMSC/VLR; and sending a confirmation message from the control node in theSAE/LTE network to the mobile terminal; and if the mobile terminal hasnot changed location areas, sending the confirmation message from thecontrol node in the SAE/LTE network to the mobile terminal.
 7. A controlnode in a System Architecture Evolution/Long Term Evolution (SAE/LTE)network, comprising: an SAE/LTE communication unit for wirelesslycommunicating with a mobile terminal operating in the SAE/LTE network; aServing GPRS Service Node (SGSN) mimic unit for mimicking the messagingfunctionality of an SGSN; and a Gs interface connection from the SGSNmimic unit to a mobile switching center/visitor location register(MSC/VLR) in a circuit-switched (CS) network; wherein the Gs interfaceconnection is configured to receive a paging message from the MSC/VLRutilizing the Gs interface, and the SAE/LTE communication unit isconfigured to send an equivalent of a CS paging request message from thecontrol node to the mobile terminal in the SAE/LTE network, theequivalent of the CS paging request message indicating the MSC/VLR hasreceived an incoming CS voice call for the mobile terminal, wherein theequivalent of the CS paging request message causes the mobile terminalto send a CS paging response directly to the MSC/VLR.
 8. The controlnode according to claim 7, wherein: the SAE/LTE communication unit andthe SGSN mimic unit are configured to forward registration informationfrom the mobile terminal to the MSC/VLR, and to forward an identifierfor the MSC/VLR to the mobile terminal.
 9. The control node according toclaim 7, further comprising: a signaling unit for receiving a requestfrom the SAE/LTE network to establish a data session with the mobileterminal; wherein the SAE/LTE communication unit is configured toforward the request to the mobile terminal for direct response to thecontrol node.
 10. The control node according to claim 8, wherein thecontrol node in the SAE/LTE network receives a tracking area updatemessage from the mobile terminal, the tracking area update messageindicating that the mobile terminal has changed tracking areas in theSAE/LTE network, and the control node further comprises: comparing meansfor determining whether the mobile terminal has also changed locationareas associated with the CS network; wherein, if the mobile terminalhas also changed location areas, the control node in the SAE/LTE networkis configured to send a location update message to the MSC/VLR; andwherein, if the mobile terminal has not changed location areas, thecontrol node in the SAE/LTE network is configured to send a confirmationmessage to the mobile terminal.
 11. A mobile terminal for receiving acircuit-switched (CS) voice call while operating in apacket-switched-only System Architecture Evolution/Long Term Evolution(SAE/LTE) network, said mobile terminal comprising: a packet-switchedcommunication protocol stack for sending and receiving messages with acontrol node in the SAE/LTE network, wherein the control node isconfigured to mimic the messaging functionality of a Serving GPRSService Node (SGSN) and is connected by a Gs interface connection to amobile switching center/visitor location register (MSC/VLR) in a CSnetwork; a circuit-switched communication protocol stack for sending andreceiving messages with the MSC/VLR in the CS network; and a controllerfor determining when to use the packet-switched communication protocolstack and when to use the circuit-switched communication protocol stack,wherein: the controller is configured to determine whether a pagingmessage received from the SAE/LTE network is an invitation toparticipate in a data session in the SAE/LTE network, or is aninvitation to accept an incoming CS voice call; wherein the controlleris configured to utilize the packet-switched communication protocolstack to send a response to a control node in the SAE/LTE network whenthe paging message is an invitation to participate in a data session inthe SAE/LTE network; and wherein the controller is configured to utilizethe circuit-switched communication protocol stack to send a responsedirectly to the MSC/VLR when the paging message is an invitation toaccept an incoming CS voice call.
 12. The mobile terminal according toclaim 11, wherein the paging message is an invitation to accept anincoming CS voice call, and the controller is configured to perform CScall setup procedures with the CS network to set up the CS voice call.13. The mobile terminal according to claim 11, wherein the MSC/VLR is ina combined CS/PS network that also includes a Serving GPRS Service Node(SGSN), and the mobile terminal is actively involved in a PS datasession in the SAE/LTE network when a paging message is receivedinviting the mobile terminal to accept an incoming CS voice call,wherein: the controller is configured to perform CS call setupprocedures in the combined CS/PS network to set up the CS voice call;the controller is configured to forward a routing area update messagefrom the mobile terminal to the SGSN, said routing area update messageannouncing the mobile terminal's presence in the combined CS/PS network;and the controller is configured to perform PS session establishmentprocedures in the combined CS/PS network to reestablish the PS datasession.